The similarity of late Paleozoic cyclothems in North America and Europe suggests a similarity in origin that operated widely. This has been explained by several theories variously emphasizing shifting sedimentation zones related to shifting shore lines, periodically subsiding basins, successive uplifts of the land followed by peneplanation, cycles of increasing and decreasing rainfall, oscillations of both land and basins, fluctuating sea-level related to glaciation in the Southern Hemisphere, sedimentation controlled by barriers and bars, differential compaction of various kinds of sediment, et cetera. None of these theories has gained much following.

The diastrophic control theory, favored by the writer, holds that cyclothems owe their development to repeated oscillations, each consisting of a long gradual subsidence followed by a short sharp uplift, both centering in the area from which the major part of the sediments was derived but operating also in the basins of sedimentation. It is based on generalizations concerning the cyclical repetition of similar members and their characters, the alternation of non-marine and marine strata, the asymmetrical nature of cyclothems, and the widespread development of many members which are believed to be essentially time-stratigraphic units.

The conclusions are reached that uplift and depression alternately affected both source area and basins and that these movements were synchronous and similar in both regions. At times of uplift, erosion was rapid in the source area and coarse sediments were transported across a sloping alluvial coastal plain. At times of subsidence, erosion was less active, and finer sediments were transported more slowly across a contracting more gently sloping alluvial plain into a transgressing sea.

In spite of somewhat conflicting evidence, the main source area in eastern North America is believed to have been a long narrow upland tract adjacent to the Atlantic coast. Estimates of the movements in this area indicate that oscillations probably were produced by vertically acting forces. The mechanism responsible for these movements is not known, but thermal contraction and expansion may be suggested. Calculations based on various estimates show that such an explanation is not impossible.

The diastrophic control theory does not explain all features of late Paleozoic cyclothems and many interesting problems remain for future investigation in this fascinating and complex field of stratigraphic research.